Polyurethane resins are produced by the reaction of polyols and organic polyisocyanates in the presence of a catalyst and optionally additives, such as a foaming agent, a surfactant, and a crosslinking agent. Polyurethane resins have excellent adhesion to base materials, flexibility, weather resistance, etc., and are thus widely used for applications, such as paints and adhesives for vehicles, buildings, household appliances, heavy anticorrosion and plastics paints.
Examples of catalysts used in the production of polyurethane resins include organic tin catalysts, such as dibutyltin dilaurate and tin octanoate (NPL 1). However, organic tin catalysts are highly toxic, and their properties hazardous to the environment and human bodies are problematic. There have already been movements mainly in Europe to regulate the use of organic tin catalysts in the production of polyurethane resins. There has been strong demand for alternative catalysts for organic tin catalysts.
In order to solve this problem, there is reportedly a method using an N-heterocyclic carbene as a catalyst in the polymerization reaction of an aliphatic diisocyanate and an aliphatic diol (NPL 2). However, such carbenes are generally compounds that are unstable against oxygen and water. Further, it is necessary to handle them in special equipment, such as glove boxes. Accordingly, they were not satisfactory in terms of practical aspects.
As a method for solving this problem, there is a known method that uses CO2 adducts of N-heterocyclic carbenes as thermally latent catalysts for polyurethane production (NPL 3). However, CO2 adducts of N-heterocyclic carbenes are decomposed by heat. Accordingly, when they act as catalysts, CO2 gas is generated as their decomposed product; thus, there is a problem that voids are formed particularly when they are used for paint applications. Moreover, as a result of examination, the present inventors found that CO2 adducts of N-heterocyclic carbenes are rapidly hydrolyzed at 80° C. in the presence of water (see the Evaluation Examples, provided later). In light of the above, the use of CO2 adducts of N-heterocyclic carbenes as thermally latent catalysts for polyurethane production still has problems that should be solved.
Furthermore, the CO2 adducts of N-heterocyclic carbenes disclosed in NPL 3 have bulky substituents, such as 2,4,6-trimethylphenyl groups, on two nitrogen atoms of the N-heterocyclic carbenes. Such compounds require complicated production processes, and are industrially disadvantageous.